It is known to transport pulverulent materials in the fluidized state from one point to another. A material is said to be fluidizable if it is in pulverulent form and if its grain size and cohesion are such that blown in air, even at low speed, leads to decohesion of the particles and reduction of internal frictional forces, so that the suspension thus formed behaves like a homogeneous fluid. Such materials are e.g., alumina, cement, plaster, lime, fly ash, calcium fluoride, fillers for rubber and plastic, catalysts, pulverized coal, sulfates, phosphates, metal powders, plastic materials in powder form, food products such as starches, powdered milk, flours.
The following three patents, owned by applicant, are illustrative of the state of the art.
French Patent 2 575 734, entitled "Apparatus for the distribution with a regulated flow rate of a fluidizable pulverulent material", describes an apparatus making it possible to regulate the flow rate of a fluidizable material, namely, alumina.
French Patent 2 575 680, entitled "Fluidized bed apparatus for the continuous separation of two mixed solid phases", described an apparatus making it possible to separate in a product formed by fluidizable fine particles the masses of agglomerated particles unsuitable for fluidization.
French Patent 2 391 136, entitled "Process for the autoregulation of a pneumatic transport", describes a process and an apparatus for the automatic regulation of the flow rate in a fluidized bed transportation system using no mechanical members.
The apparatus according to the invention can apply to any of the aforementioned apparatuses and processes.
The apparatus described in French Patent 2 575 734 comprises (FIG. 1) a storage tank (1) filled with alumina connected to the container (2) by a supply column (3) issuing on the side (7A) of the container (to the left in the drawing), a container (2) having in its lower part (2B), a porous fluidization wall (4) and an intake (5) for fluidization gas at a constant, adjustable pressure, column (6) in its upper part (2A) at the end (7B) opposite to that of the supply column, a balancing and degassing, and an oulet (8) for the fluidized pulverulent material on the face (7B) corresponding to the balancing column and immediately above the porous wall (4).
In the absence of fluidization gas, the pulverulent material stored in the tank (1) drops into the container (2), forming a crumbling slope (10) whose angle with the porous fluidization wall is dependent on the nature and physical state of the pulverulent material.
On supplying the fluidization gas, with the outlet (8) closed, using the pipe (5) and the regulating means (12), through the porous wall (4), the pulverulent material starts to fluidize. It rapidly falls the upper part of the container and then gradually rises in the balancing column to a height h (FIG. 2), which is a function of the fluidization pressure P.sub.f and the average density of the pulverulent material in the balancing column (6). Calculation shows and experience confirms that, when the system is in equilibrium for a given pulverulent material and outlet diameter, the material flow rate is only a function of the pressure of the fluidization gas, which provides an advantageous means for regulating said flow rate.
In reality, the fluidization pressure P.sub.f is balanced by the hydrostatic pressure due to the fluidized bed height h in the balancing column, increased by the pressure drop in the porous wall. The one-to-one relation between the fluidization pressure P.sub.f and the material flow rate consequently presupposes that the pressure drop in the porous wall does not change, i.e., there is no clogging of that wall. This is the case with preferably clean materials having a regular grain size constituting a single fluidizable phase. However, when the material to be distributed forms two solid phases, one of which phases tends to settle under the fluidization conditions, thus one phase when settled on the porous wall increases the pressure drop through the wall. Thus, for a constant fluidization pressure, there is a reduction of the fluidized material height h in the balancing column, as from the flow through the outlet (8).
This problem occurs with fresh alumina, which contains heavy firebrick particles called "sand", which are mixed with the alumina during its calcination and also in the alumina supply system for electrolytic tanks, where alumina used for trapping fluorinated gases emitted by the tanks. This alumina, containing the trapped products, tends to form compact agglomerates, referred to as "scales", which are deposited on the porous wall.